The present invention generally relates to an electrical circuit breaker and more particularly, to an electrical circuit breaker employing a bimetal.
Conventionally, in the circuit breaker employing a bimetal, the so-called contact plate or resilient plate type circuit breaker has been well known.
The known circuit breaker of the above described type includes, for example, as shown in FIG. 1, a housing 1, and a disc-like bimetal plate 3 having an arculate curved cross section and fixed at its central portion, to an end of an adjusting screw 2 threaded into a base portion of the housing 1 to extend into said housing. When a rated current is flowing, contacts 4a and 4b provided at opposite ends on one surface of the bimetal plate 3 are caused to contact, through deflection of the bimetal plate 3, corresponding contacts 7a and 7b provided at the base portions of a pair of terminal plates 5 and 6 extending outwardly from the housing 1 and connected to the power source through a load or resistance R and a switch SW, while on the other hand, if an overcurrent flows through the circuit breaker, the disc-like bimetal is caused to deflect in the opposite direction as shown in dotted lines in FIG. 1 so that the contacts 4a and 4b thereof are spaced from the corresponding contacts 7a and 7b of the terminal plates 5 and 6 for cutting off the electrical connections therebetween.
The conventional contact plate type circuit breaker as described above, however, has such disadvantages that, since the disc-like bimetal plate 3 is formed by press work, it is difficult to achieve high dimensional accuracy of said bimetal plate and therefore, adjusting mechanisms such as the adjusting screw 2 and the like are required to provide correct temperature range for the deflection of the bimetal plate 3 in the opposite direction, i.e. to provide an accurate set value against the overcurrent, thus resulting in complication of construction and consequent large size of the circuit breaker itself.
Moreover, in the known circuit breaker of the above described type, since the contact pressure is obtained by deflecting the bimetal plate 3, said contact pressure tends to deviate from product to product, and moreover, due to the fact that the contact pressure affects the deformation of the bimetal plate 3, it is extremely difficult to achieve accurate set value for the bimetal plate 3 against overcurrent.